Add checks to the image-create-via-import commmand so that it provides
better user feedback and doesn't begin the import workflow unless the
input has a chance of succeeding. Preserves backward compatibility
with the current image-create command by (1) allowing an image record
only to be created when no import-method is specified AND no data is
supplied, and (2) doing the glance-direct workflow when no import-
method is specified AND data is provided. Also adds the ability for
the import-method to be set as an env var OS_IMAGE_IMPORT_METHOD.
Change-Id: I0a225f5471a9311217b5d90ebb5fd415c369129a
Closes-bug: #1758149
Brian Rosmaita
79543a67ed